The number of times a fluorescent lamp is turned on is known to limit the lamp's operating life. This is due to damage to the lamp electrodes which occurs during the start interval with known forms of electrical and electronic ballast used to start conventional lamps. Existing ballasts will typically start a fluorescent lamp between 10,000 and 100,000 times before the lamp becomes effectively inoperable.
A conventional rapid start fluorescent lamp is turned on by applying low voltage (typically about 3.5V) across each of the two filaments or electrodes in the fluorescent lamp tube, while simultaneously applying about 300-400V between the two electrodes. Each electrode acts alternately as an anode and as a cathode. Initially, essentially no current flows through the lamp and the 300-400V applied between the electrodes is manifested as a voltage drop between the electrodes and the gas which surrounds them. Once the electrodes are heated to a sufficient temperature, thermionic emission of electrons from the electrodes begins. The thermionic emission into the gas enclosed by the fluorescent lamp tube allows current to flow between the electrodes thereby ionizing the gas within the tube. As thermionic emission continues, current through the gas increases. When each electrode is warmed up to its operating temperature and the thermionic emission has reached a sufficiently high level, the potential difference between the gas and the electrode drops to a much lower number, in the range of 3 to 4 volts. Electronic ballast is typically provided between the power supply source and the electrode to limit the increase in current in the circuit as the gas becomes increasingly ionized. In its crudest form, such ballast may constitute a simple resistor. In such case, as thermionic emission increases the current flow through the fluorescent tube between the electrodes, the current through the resistor also increases thereby increasing the voltage drop across the resistor. This operation stabilizes when most of the voltage drop from the power supply is applied across the resistor and a relatively small voltage drop is presented between the electrodes. Usually the resistor is replaced with an inductive reactor ballast having substantially the same overall effect.
Inui et al., U.S. Pat. No. 4,215,292 discloses a circuit for operating a discharge lamp. The apparatus provides a reduced arc current during a pre-heating period. The Inui et al. device reduces but does not eliminate wear on discharge lamps. Kachmarik et al., U.S. Pat. No. 5,483,125 and Chermin et al., U.S. Pat. No. 4,253,043 provides other ballast circuits which provide a reduced arc voltage during a pre-heating interval. The pre-heating interval in each case is determined by a PTC resistor. Significant voltages can be applied between the electrodes of the Kachmarik et al. and Chermin et al. lamps even during the pre-heating interval. This is especially true if the lamps are switched off and on again while the PTC resistor is hot.
Rudolph, U.S. Pat. No. 5,583,399 shows a ballast for a fluorescent lamp in which arc voltage is reduced until after the filaments in a connected lamp become hot. The filaments are then disconnected. Disconnecting the filaments allows a resonant circuit to produce high voltage to fire the lamp. The Rudolph design does not provide continuous heater current. Lau, U.S. Pat. No. 5,444,333, shows another ballast circuit in which the heater current is off during normal operation.
Hoeksma, U.S. Pat. No. 4,988,920 discloses a power circuit for a lamp in which arc current is supplied by the secondary winding of a transformer. The arc current is disconnected until after filaments in a connected lamp are warmed up. The Hoeksma circuit appears to be primarily intended for use in devices such as photocopiers and the like. Hoeksma does not include a timer for delaying the application of arc current for a predetermined time period.
Nilssen, U.S. Pat. No. 4,949,015 shows a bridge inverter ballast which has first and second pairs of switching transistors connected to self-oscillate. When the second pair of transistors self-oscillate they generate voltage to power the main arc current in a lamp. Self oscillation of the first pair of transistors powers thermionic cathodes in a lamp. Self oscillation of the second pair of transistors is delayed until the cathodes have heated up. While the cathodes are heating up and the second set of switching transistors is not self-oscillating significant transient potentials may be applied between the lamp cathodes. Such potentials will be caused by resonances, reflections and standing waves excited by the fast square waves which are omnipresent in this design.
Damage to the electrodes occurs when they are acting as cathodes at the early stage of warm up of the electrodes when high voltage is applied to the circuit. At that stage, there is a high potential difference between each electrode and the gas surrounding it. Although the initial application of high voltage to the electrodes is brief (usually in the order of 0.2 seconds), it nonetheless causes the field emission of electrons from the electrode creating a localized plasma. Any positive ions that may be present in the area when the electrode is acting as a cathode are accelerated by the voltage drop and crash violently into the electrode thus causing damage known as sputtering damage.
In a prior art approach used in association with line voltage power sources and know as "preheat" lamps, a switch is placed in the series connection between the two electrodes and an inductor is placed in series connection between the power source and one of the electrodes. The switch is manually depressed for a period of time before being released. During the depressed or closed period, the current flows through the electrodes thereby heating them up. When the switch is released, the collapse of the magnetic field in the inductor induces a very high voltage between the two electrodes ionizing the gas in the lamp and initiating the arc discharge that characterizes normal operation. A current is then established through the gas, following which the voltage across the inductor falls to reasonable levels. A disadvantage of this approach is arcing which occurs across the switch. In such a system, sputtering damage occurs due to the high voltage drop between the electrodes which occurs during the noisy, arcing period of the closing of the manual switch, during which time the electrodes are still cold.
It is therefore an object of the invention to provide a fluorescent lamp start up control circuit which minimizes sputtering damage to the electrodes and thereby increases the number of turn ons to which the lamp may be subjected.